Recently I milled up a river birch and over the last few days it has started to accumulate green mold spots on it. I have the slabs stored in my garage stickered up and there is no humidity or condensation along with it being in a well ventilated area.

Any tips or suggestions would be a blessing as I would like to work on this wood.

From a technical viewpoint, mold is a fungal activity. To be active, fungi require moisture, food, warm temperatures and oxygen. Eliminate one of the four to control their growth. Nothing we can do to eliminate the sugars in the wood near the bark, or oxygen or temperature. So, our only control, other than poisoning their food supply at the surface BEFORE they become active, is to eliminate moisture...basically under 22% MC at the surface will not support their growth. Once a fungus gets beyond the surface, it becomes harder to control because we need to eliminate the moisture deeper within the wood. Or stated another way, it is easier to prevent the initial attack than to prevent or contro damage once they have started.